Generally described, computing devices and communication networks can be utilized to exchange information. In a common application, a computing device can request content from another computing device via a communication network. For example, a user at a personal computing device can utilize a software browser application, typically referred to as a browser, to request a web page from a server computing device via the Internet. In such embodiments, the user computing device can be referred to as a client computing device and the server computing device can be referred to as a content provider.
A web page or other network-accessible content item may be associated with a number of embedded resources, such as images or videos to be displayed, style sheets to control formatting, or scripts to be executed. A browser typically processes embedded resource identifiers to generate requests for the items. Embedded resources may contain references to other embedded resources, and so on. In many cases, the web page may also contain one or more links to other web pages, which are also associated with a number of embedded resources. When a user activates a link to another web page, a new request is generated for the linked web page, and the retrieval and processing begins again.